Semiconductor assembly with solder material layer and method for soldering the semiconductor assemly

ABSTRACT

In a semiconductor assembly with a solder material layer and a method for soldering the semiconductor assembly, a silicon semiconductor body with a diffusion barrier layer is provided with a solder material layer, preferably a tin layer. The semiconductor body is then applied to a metal carrier plate and is directly soldered to the carrier plate by heating to temperatures to above 250° C., i.e. without further additions.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a semiconductor assembly with a solder materiallayer and a method for soldering the semiconductor assembly, in which asemiconductor body formed of silicon is soldered to a metal carrierplate through a sequence of metal layers which, as seen from the silicontoward the carrier plate, includes an aluminum layer and a diffusionbarrier layer before soldering.

Such semiconductor bodies are installed in semiconductor components,especially in power semiconductor components, which are on the market inlarge amounts. The sequence of metal layers as a rule includes analuminum layer which is disposed on a silicon semiconductor body. Thealuminum layer adheres well to silicon and forms a perfect ohmiccontact, especially with p-doped silicon. According to the prior art,disposed on the aluminum layer is a diffusion barrier layer formedmostly of titanium or chromium which acts as an adhesion promoter andrearside barrier between a nickel layer disposed on the diffusionbarrier layer and the aluminum layer. In the prior art, a noble metallayer is applied to the nickel layer directly or on a thin titaniumlayer that follows and serves for adhesion improvement. The noble metallayer is mostly formed of silver, gold or palladium and serves as anoxidation protection for the nickel layer.

In the actual soldering process a solder disk is conventionally placedbetween the thus metallized rearside of the semiconductor body and themetal carrier plate which usually is formed of tin or flux. In thesoldering process, the solder layer placed between the carrier plate andthe silver layer of the semiconductor body then melts, whereby thesilver layer is then dissolved and the nickel layer that follows beginsto be dissolve from the solder material and the solder connection isproduced.

However, that generally known soldering process has great disadvantages.On one hand, the different coefficients of thermal expansion of nickeland the silicon semiconductor body cause mechanical tension which leadto high wafer bending (bending >1000 μm) especially in the case of thinsemiconductor bodies (thickness ≦250 μm).

This leads to difficult handling of the wafer which leads to increasederrors in cassette positioning and which leads to increased danger ofbreakage during treatment of the wafer. That problem was heretoforesolved by trying to minimize the thickness of the nickel layer to such adegree that the soldering still exhibited a sufficient adhesivestrength. Regardless of reduced thickness of the nickel layer (≈1 μm),disc deformations of between 700 and 2000 μm still occur duringproduction, which lead to the above-mentioned problems.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a semiconductorassembly with a solder material layer and a method for soldering thesemiconductor assembly, which overcome the hereinafore-mentioneddisadvantages of the heretofore-known devices and methods of thisgeneral type and which make it possible to metallize a siliconsemiconductor body in such a way that disc deformation can be reducedsharply without suffering a loss of adhesion stability on carriermaterials.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a semiconductor assembly, comprising asilicon semiconductor body; a metal carrier plate; and a sequence ofmetal layers for soldering the semiconductor body to the carrier plate,the sequence of metal layers before soldering including an aluminumlayer disposed toward the semiconductor body, a diffusion barrier layer,and a solder material layer applied on the diffusion barrier layer anddisposed toward the carrier plate.

In accordance with another feature of the invention, the solder materiallayer is a tin layer or a lead layer or a gallium layer.

In accordance with a further feature of the invention, the diffusionbarrier layer has a thickness of approximately 50 nm.

In accordance with an added feature of the invention, the soldermaterial layer has a thickness of less than 3000 nm.

With the objects of the invention in view there is also provided amethod for soldering a semiconductor body onto a metal carrier plate,which comprises soldering the semiconductor body on the metal carrierplate in such a way that the semiconductor body is applied to thecarrier plate and directly soldered to the carrier plate by heating totemperatures above approximately 250° C., i.e. without the addition offurther soldering materials and soldering flux.

This creates an almost tension-free solder layer which leads tosubstrate bendings of less than 300 μm. Furthermore, the adhesionstability of the soldered semiconductor body on the carrier plate isvery high, i.e. in ranges of more than 30 Mpa. Additionally, no furtherembrittlement or decay due to temperature-alternating stress occurs,since the metallic phases are only thinly defined. As a whole, theproblems in handling the wafer and the danger of breakage are thereforeeliminated.

Furthermore, a distinct cost reduction can be realized with themetallized semiconductor bodies according to the invention and themethod according to the invention, since the costs of materials can bedecreased due to the omission of an oxidation protection layer and theproduction rate can be increased. Additionally, the method according tothe invention is especially friendly to the environment because cleaningsteps with CFC-solvents are eliminated due to the freedom from solderingflux.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a semiconductor assembly with a solder material layer and a methodfor soldering the semiconductor assembly, it is nevertheless notintended to be limited to the details shown, since various modificationsand structural changes may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE of the drawing is a fragmentary, diagrammatic, sectional viewof a semiconductor assembly according to the invention.

BRIEF DESCRIPTION OF THE DRAWING

Referring now in detail to the single FIGURE of the drawing, there isseen a layer sequence of metals before soldering. According to theinvention, an aluminum layer 3 which is applied on a semiconductor body1 adheres well on the silicon and forms a perfect ohmic contact withp-doped silicon. A diffusion barrier layer 4 which is disposed on thealuminum layer 3 is formed of chromium or titanium with a thickness ofapproximately 50 nm. This layer is either directly sputtered on thealuminum layer 3 or directly vacuum metallized. A non-illustratedadhesion-promoting layer, for example Cu, is applied on the diffusionbarrier layer 4 or a solder material layer 5 is directly applied. Thesolder layer 5 is either formed of tin, gallium or lead and has athickness of 1000 to 3000 nm. When using tin, the thickness usuallyamounts to 2700 nm. The thus metallized silicon semiconductor body 1 isthen pressed on a metal carrier plate 2 which usually is formed ofcopper and is connected to the same in a protective gas atmosphere or ina vacuum at approximately 300° C. In this way a metallurgical connectionbetween the diffusion barrier layer 4, the solder material layer 5 andthe carrier plate 2 is created, which is stable up to a temperature ofapproximately 450° C.

A product of high quality is created through the use of the methodaccording to the invention since the described sequence of layersproduces an especially good mechanically and electrically stablecontact. The method can be described as being very advantageous ascompared to the prior art in technological as well as economical andecological respects.

Technologically, the method according to the invention opens thepossibility of further reducing the thicknesses of the siliconsemiconductor substrate which leads to an improvement in the porousquality of the power semiconductor components. Economically, theadvantage lies in the fact that costs of the material can be decreasedand that the production rate can be increased. Finally, the ecologicaladvantage of the method according to the invention lies in the fact thatthe solder process is carried out without solder flux and that cleaningsteps with CFC-containing solvents can be avoided.

We claim:
 1. A semiconductor assembly, comprising:a siliconsemiconductor body; a metal carrier plate; and a sequence of metallayers for soldering said semiconductor body to said carrier plate, saidsequence of metal layers before soldering consisting essentially of analuminum layer disposed toward said semiconductor body, a diffusionbarrier layer selected from the group consisting of chromium andtitanium, and a solder material layer applied on said diffusion barrierlayer and disposed toward said carrier plate, said solder material layerhaving a thickness of less than 3000 nm.
 2. The semiconductor assemblyaccording to claim 1, wherein said solder material layer is formed of amaterial selected from the group consisting of tin, lead and gallium. 3.The semiconductor assembly according to claim 1, wherein said diffusionbarrier layer has a thickness of approximately 50 nm.